On Tue, Aug 29, 2017 at 5:58 PM, David Miller <da...@davemloft.net> wrote: > From: Tom Herbert <t...@quantonium.net> > Date: Tue, 29 Aug 2017 16:27:07 -0700 > >> For UDP socket lookup functions, and associateed functions that take an >> skbuf as argument, declare the skb argument as constant. >> >> One caveat is that reuseport_select_sock can be called from the UDP >> lookup functions with an skb argument. This function temporarily >> modifies the skbuff data pointer (in bpf_run via a pull/push sequence). >> To resolve compiler warning I added a local skbuf declaration that is >> not const and assigned to the skb argument with an explicit cast. >> >> Signed-off-by: Tom Herbert <t...@quantonium.net> > > Please don't do this. > > If reuseport_select_sock() modifies anything in the SKB, especially > skb->data, it infects the entire call chain. So you can't mark it > const in this family of calls. > reuseport_select_sock calls run_bpf that calls pskb_pull to "temporarily advance data past protocol header" and it calls bpf_prog_run_save_cb which takes non-constant skb argument. This is the only instance in all the udp lookup functions where non-constant is needed. It's logical that constant skbuf makes sense for socket lookup-- I doubt any caller would expect the skbuf to be modified as a side effect. It's also an implicit characteristic since reuseport_select_sock may just clone the socket before calling BPF.
The problem is that all the flow dissector functions operate on const skbs (again that's logical :-) ). So if we want to be able to call lookup functions or even BPF to do flow dissection, then I think something needs to change. I really don't want to unconsitify the flow dissector functions. We could just always do the skb before calling BPF, but I suppose that is a potential performance hit. Is there a better way to resolve this? Thanks, Tom
